Unusual seasonal patterns and inferred processes of nitrogen retention in forested headwaters of the Upper Susquehanna River

Abstract

Atmospheric deposition contributes a large fraction of the annual nitrogen (N) input to the basin of the Susquehanna River, a river that provides two-thirds of the annual N load to the Chesapeake Bay. Yet, there are few measurements of the retention of atmospheric N in the Upper Susquehanna's forested headwaters. We characterized the amount, form (nitrate, ammonium, and dissolved organic nitrogen), isotopic composition (d 15 N-and d 18 O-nitrate), and seasonality of stream N over 2 years for 7-13 catchments. We expected high rates of N retention and seasonal nitrate patterns typical of other seasonally snow-covered catchments: dormant season maxima and growing season minima. Coarse estimates of N export indicated high rates of inorganic N retention ([95%), yet streams had unexpected seasonal nitrate patterns, with summer peaks (14-96 lmol L -1 ), October crashes (\1 lmol L -1 ), and modest rebounds during the dormant season (\1-20 lmol L -1 ). Stream d 18 O-nitrate values indicated microbial nitrification as the primary source of stream nitrate, although snowmelt or other atmospheric source contributed up to 47% of stream nitrate in some March samples. The autumn nitrate crash coincided with leaffall, likely due to in-stream heterotrophic uptake of N. Hypothesized sources of the summer nitrate peaks include: delayed release of nitrate previously flushed to groundwater, weathering of geologic N, and summer increases in net nitrate production. Measurements of shale d 15 N and soil-, well-, and streamwater nitrate within one catchment point toward a summer increase in soil net nitrification as the driver of this pattern. Rather than seasonal plant demand, processes governing the seasonal production, retention, and transport of nitrate in soils may drive nitrate seasonality in this and many other systems.

Nutrient Uptake and Mineralization during Leaf Decay in Streams – a Model Simulation

“…MULHOLLAND et al (1985) demonstrated that phosphorus uptake ) reported very rapid uptake of ammonium in another stream at Coweeta based on measurements made during peak leaf fall, and VALETT et al (2008) found that nitrate uptake in streams draining deciduous forests was greatest when temperatures were low but leaf standing crop in the streams was high. While these and other studies (e.g., NEWBOLD et al 2006;HOELLEIN et al, 2007;GOODALE et al, 2009) provide strong evidence for the connection between nutrient uptake and decaying leaves, we know of no published evidence documenting elevation of water column nutrient concentrations resulting from decaying leaves. HOWARTH and FISHER (1976) found increases in water column nitrogen and phosphorus during leaf decay in laboratory chambers under elevated nutrient conditions, but we are not aware of similar results for natural streams.…”

Mechanisms driving the seasonality of catchment scale nitrate export: Evidence for riparian ecohydrologic controls

“…Importantly, at 0.015% N, bedrock N percentages are approximately 2 orders of magnitude lower than the bedrock found to be geologic sources of stream N by Holloway and Dahlgren [1999]. The low rock N content at Pond Branch was within the range of other formations underlying watersheds whose stream waters also exhibit summer peaks in N concentrations [Mulholland and Hill, 1997;Goodale et al, 2009] where geogenic sources have been ruled as unlikely. We therefore conclude that geogenic sources are unlikely drivers of the observed seasonal N patterns at Pond Branch.…”

Isotopes for improved management of nitrate pollution in aqueous resources: review of surface water field studies

“…In several isotope studies, it has been shown that atmospheric nitrate has no large direct contribution to surface water nitrate in forested watersheds (Burns et al 2009;Burns and Kendall 2002;Haberhauer et al 2002;Hales et al 2007;Ohte et al 2004;Piatek et al 2005Piatek et al , 2009, whilst δ 18 O-NO 3 − data suggested that nitrate is mainly derived from nitrification (Barnes et al 2008;Burns et al 2009;Burns and Kendall 2002;Goodale et al 2009;Hales et al 2007;Pardo et al 2004, Piatek et al 2005. Most atmospheric nitrate is processed through biota and then nitrified before entering the stream (Burns and Kendall 2002).…”

“…With a dual-isotope approach, it was shown that in two forested watersheds, Biscuit Brook and Buck Creek, nitrified soil nitrogen was the main nitrate source (Burns et al 2009). Microbial nitrification was identified by means of δ 18 O-NO 3 − as the primary source of stream nitrate (Goodale et al 2009). In some of the samples obtained during snowmelt, atmospheric nitrate contributed up to 47% to the stream nitrate (Goodale et al 2009).…”